Interpretive Summary: Corn germplasm lines with resistance to feeding by fall armyworm larvae have been developed and released by USDA-ARS. This investigation was undertaken to identify the factors associated with resistance in Mp708 to feeding damage by fall armyworm. Mp708 has moderately high constitutive expression of jasmonic acid and other octadecanoid compounds unlike the susceptible line Tx601 which only produces these compounds in response to larval feeding. Mp708 is primed to respond rapidly to fall armyworm feeding. Further, analysis of the volatiles released by Mp708 and Tx601 before and after exposure to fall armyworm demonstrated the presence of (E)-beta-caryophyllene, a terpenoid associated with resistance in Mp708. Identifying specific factors associated with resistance will facilitate their incorporation into commercial corn hybrids that sustain less damage from fall armyworm. This could reduce losses to fall armyworm and the need for insecticides.

Technical Abstract:
Maize (Zea mays L.) is an important agricultural crop. Various insect pests such as those in the order of Lepidoptera frequently feed on young maize plants and pose a significant threat to plant development and survival. To deal with this problem, maize generates a wide variety of responses to attack by pests, from activation of wound-response pathways such as jasmonic acid (JA) biosynthesis to the release of volatile compounds. Several maize lines have been developed that show resistance to the larvae of the fall armyworm [Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae)]. Mp708, an inbred line resistant to feeding by fall armyworm, was developed through traditional breeding methods, but the mechanisms of resistance are not completely understood. Mp708 has been shown to have a moderately high constitutive expression of JA and other octadecanoid compounds prior to infestation by fall armyworm. On the other hand Tx601, a genotype susceptible to feeding by fall armyworm, activates JA pathway only in response to feeding, suggesting that Mp708 is “primed” to respond swiftly to attack. Current research indicates that fall armyworm larvae show a lack of preference to feeding on Mp708, leading to the hypothesis that volatiles released by the plant may also play an important role in its resistance. Analysis of the volatiles released by the resistant and susceptible lines in the presence and absence of the fall armyworm was conducted using Solid Phase Microextraction (SPME) in conjunction with gas chromatography mass spectrometry. We have demonstrated the presence of (E)-ß-caryophyllene, a terpenoid compound associated with resistance, in Mp708. In addition, 4-day-old fall armyworm larvae show a marked preference for Tx601 whorl tissue over Mp708 tissue. Identifying specific volatiles correlated with resistance to fall armyworm could lead to the integration of these traits into commercial maize hybrids.